1. Field of the Invention
The present invention relates to a liquid crystal display element used for liquid crystal display devices displaying various kinds of images. More particularly, the present invention relates to a liquid crystal display element of high-aperture rate produced by a simple method at low cost.
2. Description of the Prior Art
Liquid crystal display devices have been widely used as display devices which can be produced in light-weight, small-size, and thin shapes. Among them, active matrix type liquid crystal display devices according to a twisted nematic mode (TN mode) are widely known to have high contrast and high image quality with low driving voltage and low power consumption.
Liquid crystal display elements according to the above general TN mode a re constructed as follows: two glass substrates each provided with a polarizing plate, a transparent electrode, and an orientation film are positioned opposedly at an interval such that the orientation direction of each orientation film differs 90.degree. from the other in order to twist nematic liquid crystals, which are provided between the substrates, by 90.degree.. In other words, as is shown in FIG. 6A, a polarized light beam transmitted through a substrate 1 having a polarizing plate alters its polarization direction .alpha. by 90.degree. by proceeding along liquid crystals 3 twisted at 90.degree. so that the light beam can pass through another substrate 2 having a polarizing plate of which polarization direction is .beta.. As a result, the liquid crystal display element is in a bright state. Meanwhile, by applying a voltage between the substrates 1 and 2, the liquid crystals 3 are oriented along the resulting electric field, as is shown in FIG. 6B. Thus, the polarization direction .alpha. of the polarized light beam does not change and the light beam is shaded by the polarizing plate of the substrate 2, resulting in a dark state of the liquid crystal display element.
Currently dependency on angle of view is a problem in liquid crystal display elements of the above TN mode. FIG. 7 shows general dependency of liquid crystal display elements of TN mode on angle of view. The shaded portion in FIG. 7 indicates a region in which contrast (CR) is not less than 10. It is apparent from FIG. 7 that although visibility from the lateral direction is excellent in liquid crystal display elements of TN mode, that from the vertical direction, and particularly from above, is extremely inferior. Reasons for the above will be explained below.
As is shown in FIGS. 6A and 8A, when all liquid crystal molecules lie flat and are oriented in the same direction, refractive indexes, i. e., n1 and n2 shown in FIG. 8A, are not largely affected by incident directions of light beams entering into and outgoing from the liquid crystal layer and have almost the same value. Therefore the dependency on angle of view is not remarkable. However, when liquid crystal molecules are oriented along an applied electric field such that the tilt angle of each liquid crystal molecule differs from the others, as is shown in FIG. 6B or 8B, the refractive indexes n1' and n2' are different depending on the incident angles of transmitting light beams because liquid crystal molecules have different refractive indexes for the major axis direction and the minor axis direction. As a result, in a normally white display mode the transmittance under applied voltage is largely affected by dependency on angle of view and thus contrast shows significant changes depending on angle of view.
The inventors of the present invention describe epoch-making liquid crystal display elements for solving the problem of viewing from above, in specifications of Japanese Patent Application Nos. 7-1579 and 7-306276.
According to techniques disclosed in the above patent applications, electrodes for driving liquid crystals are not provided for each of the upper and lower substrates holding the liquid crystals therebetween. Two types of linear electrodes 12 and 13 each having different polarities are provided at separate positions only for the lower substrate (the first substrate) 11 shown in FIG. 9 and no electrode is provided for the upper substrate (the second substrate) 10. By applying a voltage, liquid crystal molecules 36 are oriented along the directions of electric fields generated between the linear electrodes 12 and 13. In more detail, the linear electrodes 12 are connected by a base wire 14 to form a comb-shaped electrode 16 and the linear electrodes 13 are connected by a base wire 15 to form a comb-shaped electrode 17; the linear electrodes 12 and 13 of the comb-shaped electrodes 16 and 17 are positioned alternately so as not to contact each other; and a switching element 19 connects to the base wires 14 and 15. Furthermore, as is shown in FIG. 11, an orientation film is formed on the liquid crystal side of the upper substrate 10 to align the liquid crystal molecules 36 in the .beta. direction, another orientation film is formed on the liquid crystal side of the lower substrate 11 to align the liquid crystal molecules 36 in the .gamma. direction parallel to the .beta. direction, and conventional polarizing plates are provided for the substrates 10 and 11.
According to the above structure, the liquid crystal molecules 36 are homogeneously oriented in the same direction when no voltage is applied between the linear electrodes 12 and 13, as is shown in FIGS. 11A and 11B. A light beam transmitted through the lower substrate 11 is polarized in the .alpha. direction by the polarizing plate, passes through a layer of the liquid crystal molecules 36, and then reaches the polarizing plate of the upper substrate 10, which polarizing plate has a polarization direction .beta. different from the direction .alpha.. The light beam is thereby shaded by the polarizing plate of the upper substrate 10 and unable to pass through the liquid crystal display element, thereby rendering the liquid crystal display element in a dark state.
When a voltage is applied between the linear electrodes 12 and 13, among the liquid crystal molecules 36, those adjacent to the lower substrate 11 are aligned perpendicular to the longitudinal direction of the linear electrodes 12 and 13. The nearer a liquid crystal molecule is located to the lower substrate, the more strongly this phenomenon is observed. In other words, lines of electric force perpendicular to the longitudinal direction of the linear electrodes 12 and 13 are generated so that the longitudinal axes of the liquid crystal molecules, 36 oriented in the .gamma. direction are altered perpendicular to the .gamma. direction by controlling the force of the electric field which is stronger than that of the orientation film.
Therefore, 90.degree. twisted orientation is achieved by applying a voltage between the linear electrodes 12 and 13, as is shown in FIGS. 12A and 12B. Under this condition, the polarization direction of polarized light beams transmitted through the lower substrate 11 and polarized in the .alpha. direction is converted by the twisted liquid crystal molecules 36 such that the polarized light beams are allowed to pass through the upper substrate 10 having a polarizing plate with polarization direction .beta., thereby exhibiting a bright state.
FIGS. 13 and 14 show a structure in which the linear electrodes 12 and 13 are applied to a practical active matrix type liquid crystal driving circuit.
In the structure shown in FIG. 13, on a transparent substrate 20 such as a glass substrate, a metallic gate electrode 21 and metallic first linear electrodes 22 are provided separately and in parallel with each other. A gate insulating film 24 is formed to cover these electrodes, a source electrode 27 and a drain electrode 28 are formed on a portion of the gate insulating film 24 corresponding to the gate electrode 21, a semiconductor film 26 is provided between the source electrode 27 and the drain electrode 28, and a metallic second linear electrode 29 is formed on the gate insulating film between the first linear electrodes 22. FIG. 14 is a plan view of the structure shown in FIG. 13. Gate wires 30 and signal wires 31 are formed on the transparent substrate 20 according to a matrix pattern. The gate electrode 21 leading to the gate wire 30 is provided at a corner of each region surrounded by the gate wires 30 and the signal wires 31. The second linear electrode 29 connects to the drain electrode 28 via a base wire 33 and is provided between the first linear electrodes 22 which are connected through a base wire 34. The base wires 33 and 34 overlap each other with the gate insulating film 24 shown in FIG. 13 therebetween so as to ensure capacitance.
In the above structure, lines of electric force produced by electric fields are formed along the directions of the arrows a shown in FIGS. 13 and 14. Thus, the liquid crystal molecules 36 are oriented in a manner shown in FIG. 13.
However, according to the above structure of a liquid crystal display element, the first linear electrodes 22 and the second linear electrode 29 are light-shading metallic electrodes, thus the shading area tends to be exceedingly large and the aperture rate of the resulting liquid crystal display element disadvantageously decreases.
Moreover, when the gate electrodes 21 are provided below the semiconductor film 26, overlapping portions between the gate electrodes 21, source electrodes 27, and drain electrodes 28 increase, resulting in a parasitic capacitance via the gate insulating film 24 formed therebetween. Thus, capacitance at the time of liquid crystal driving differs from the desired value, which fact could incur display-quality deterioration.